1. Field of the Invention
The present invention relates to a method of producing a diffraction grating having a first portion and a second portions of which phases are inverted each other, in a first member.
2. Related Background of the Invention
In a semiconductor laser device of DFB (Distributed Feedback) type, a diffraction grating is formed between semiconductor layers. The DFB semiconductor laser device can generate laser light with a wavelength according to a period of the diffraction grating. Phases of a first portion and a second portion in the diffraction grating is inverted each other, whereby the DFB semiconductor laser device can generate laser light with a single mode. In the patent document 1 which is Japanese Patent Application Laid-open No. Hei 5-142406, a method of producing a diffraction grating having a phase inverted portion described above is disclosed.
According to the method of producing a diffraction grating disclosed in the patent document 1, a diffraction grating is produced as follows. First, by forming a mask on a first region on a main surface of a first member, which is originally flat, and etching the first member, a difference in level is formed between the first region and a second region of the first member. Next, by applying a resist to the first region and the second region of the first member and performing exposure and development of the resist, masks which are arranged at constant period is formed. By etching the first member by use of the masks, depressions and projections arranged at a constant period is formed in these regions. Furthermore, a second member with lower etching rate than that of the first member is formed on the first region and the second region of the first member and then, etching is performed. Due to the etching, depressions and projections inverted with respect to the depressions and projections formed in the previous etching process are formed in the first region. In this way, it is possible to produce a diffraction grating having a first portion and a second portion of which phases are inverted each other, in a first member.